2 Answers
2

How are you using the external sound card/audio interface with the iPad(s)? A better answer will require a more detailed explanation of our setup, including model numbers or at least what kind of connectors each component provides.

The simple answer is if you can plug source A into speaker B and it works well enough for you without a mixer, then no you don't.

If you only had a single laptop, I would imagine your external audio interface would have a decent volume knob and the appropriate pro-quality connectors (likely TRS 1/4") to supply your powered speakers, so you wouldn't need a mixer.

You only really need a mixer if you have two distinct audio sources that you need to balance. If you are using multiple laptops and iPads simultaneously and don't want to fiddle with cables when you switch between them, then yes, you will probably need a mixer with enough stereo channels to handle each of your sources.

This is in a DJ context, so I'm presuming you're talking about a DJ-style mixer with a crossfader, rather than a more traditional line mixer.

You've got at least two audio sources (at least one laptop, at least one iPad), so if you want to play them at the same time, you'll need some kind of mixer.

However, if you plan to play only one device at a time (presumably switching the cable from one to the other) then the only reason you'd need a mixer is if you want to use some specific feature of that mixer for your performance. You could, for example, put the laptop and the iPad on either side of the mixer, and alternate playing songs on each one while using the crossfader to blend/cut between them.

However, you may be using DJing software that is handling this, and if that's the case, then the mixing is already done in software and no hardware mixer is necessary.